1. Field of the Invention
The present invention relates to a method for fabricating a gyroscope and particularly, to a method for fabricating a gyroscope using an SMS (Si-Metal-Si) wafer and a gyroscope fabricated by the same.
2. Description of the Related Art
A gyroscope is generally known as a sensor for measuring an angular velocity of a moving object, and it plays a very important role in an inertial navigation system for measuring posture and location of moving objects such as vessels, airplanes, missiles, etc.
Recently, a micro gyroscope using the MEMS (Micro Electro Mechanical System) technology has become available with small size, high sensitivity, and low price. The micro gyroscope is used for various applications including automobiles, electronic appliances, and information telecommunications.
The micro gyroscope operates in the following way in which: when a 2-degree-of-freedom movable structure in x, y directions, being applied with electrostatic force in a lateral direction, is given rotational force with a vertical rotational axis (z direction), Coriolis force of up-and-down direction (y direction) is applied to the movable structure, whereby the movable structure is vibrated in an up-and-down direction, and the sensor circuit detects an amount of vibration in units of capacitance. The movable structure is vibrated in a vacuum state to facilitate the vibration of the movable structure using a low voltage and improve sensitivity.
In the meantime, the movable structure, which is a principle element of the micro gyroscope, may comprise a cantilever with one part being spaced away from a substrate and a bridge forming a space by floating above a central portion while fixed at both ends of the substrate. In forming the movable structure, a method of etching of a sacrificial layer is used. For a sacrificial layer, an oxidation film having a large etching selective ratio with respect to silicon, is used.
FIG. 1A through FIG. 1D illustrate the general structure of the micro gyroscope and fabricating method thereof. In the drawings, the reference numerals 10, 20, 30 represent a semiconductor wafer, a movable structure, a cap, respectively.
For the semiconductor wafer 10, an SOI (Si On Insulator) wafer, such as a SOI wafer comprising an oxidation film 13 of a predetermined thickness interposed between a first wafer 11 and a second wafer 12 as shown in FIG. 1A, is used, which is considerably expensive.
A first step for fabricating the micro gyroscope comprises preparing or fabricating the semiconductor wafer 10 having the foregoing structure, and a second step comprises forming the movable structure 20 on the relevant portion of the semiconductor wafer 10.
The movable structure 20 is formed by patterning the relevant portion of the first wafer 11 of the semiconductor wafer 10 through the photolithography process and performing dry etching process. One example of the movable structure 20 formed by such process is shown in FIG. 1B.
After the movable structure 20 is formed, the process for wet etching of the relevant portion of the oxidation film 13 is performed to float the movable structure 20 above from the semiconductor wafer 10, as shown in FIG. 1C.
Finally, as shown in FIG. 1D, the cap 30 is anodic-bonded on the upper part of the semiconductor wafer 10 so that the movable structure 20 is sealed, whereby the portion where the movable structure 20 is situated becomes vacuum. The cap 30 is made of glass.
However, the foregoing method for fabricating the general micro gyroscope has the problems as follows and problem-solving is required.
Firstly, since the high priced SOI wafer is used, fabricating cost is increased.
Secondly, since the oxidation film 13 is used for a sacrificial layer, an undesirable notch 41 (FIG. 1B) is generated at the terminals of the movable structure 20 from the dry etching process of the first wafer 11 for formation of the movable structure 20, and also an undesirable undercut 42 (FIG. 1C) is generated from the wet etching process of the oxidation film 13 for floating the movable structure 20. Furthermore, the agglutination phenomenon is generated during the wet etching processing in which the movable structure 20 sticks to the second wafer 12. Such agglutination phenomena are usually generated during the process for anodic-bonding the sealing cap 30, and the notch 41, the undercut 42, and the agglutination phenomenon not only generate deterioration in the characteristics and performance of the gyroscope, but also cause reduction in product yield.
Thirdly, since materials of the semiconductor wafer 10 and the cap 30 are different from each other, stress due to the difference in thermal expansion coefficient is generated, and therefore, the structure gets unstable.